In radio systems, where information is transferred on a radio path, the desired signal is impaired by interference caused by other users or systems and noise which sum into the signal. Therefore, radio systems employ different diversity methods to increase the coverage area and/or capacity of the system. One of them is spatial diversity, which is obtained using an array antenna comprising a plural number of antenna elements that are physically separate from each other. The received signals are combined in diversity receivers using a suitable combining method.
The task of combining is thus to combine signal components received with different antenna elements and to minimise the effect of noise and interference on the desired signal. Current receivers are based on statistical signal models the accuracy of which cannot be relied on in all situations. A known combining method, which can reduce the impact of noise and interference, is Maximal Ratio Combining (MRC) method. However, this method supposes that the interference and noise in each antenna element are independent of other antenna elements, i.e. they are white. This is not always true in actual cellular radio networks, in particular. For example, in many cases even only a few high-power signals may cause interference upon reception that affects all the antenna elements, i.e. the interference in the antenna elements is coloured. Another known combining method is Interference Rejection Combining (IRC). IRC does not contain assumptions about whether interference and noise correlate with antenna elements. However, neither of these known methods has optimal interference rejection performance.
A proposed improvement is Space Time Interference Rejection Combining (STIRC). In this method, the received signal is oversampled, i.e. more than one sample is taken from each received symbol. It has been shown that the interference rejection capability of STIRC is 10 to 20 dB better than the capability of an equivalent MRC method. However, one problem with STIRC is that there can be significant amount of loss when the system is tested in noise-limited environments using weak channel codes. Compared with MRC, the STIRC algorithm may show 0.5-1 dB degradation in sensitivity simulations.
Receiver sensitivity is a key performance criterion in network planning. Good base station sensitivity can allow lower mobile station transmission power, thereby reducing overall interference, allowing better mobile station battery life, and hence lowering the number of sub-cells in coverage limited rural areas. However, increasing capacity by deploying smaller cells may increase co/adjacent channel interference. Therefore, one of the issues in finding a combiner solution is to obtain an algorithm that maintains the existing sensitivity performance but does not significantly affects the interference performance.
There have been proposals which combine MRC and STIRC methods. A suggested mechanism shown in FIG. 1, is to switch between MRC and STIRC according to measured noise co-variance and variance terms. Signals are received and noise co-variance and variance terms are measured and estimated in block 100. In block 102, decision on the combining method to be used is made. On the basis of the decision, received signals are forwarded to respective combining unit 104 or 106. This solution provides a hard switch between two algorithms and may provide give a reasonable solution provided that there is no overlap in the noise- and interference-limited regions on the used decision boundary. In practice, considering all channel conditions, obtaining an optimum decision boundary is a difficult task, which in turn causes significant interference removal loss. For example, it has been shown that the above solution where the best decision boundary is optimised against all possible channel conditions provides 3-5 dB interference losses while focusing the sensitivity gain closer to MRC. Thus, almost half of the STIRC interference gain is lost due to the extra complexity of the two different combiners.